The sensitivity of solid and liquid propellants to electrical discharge is well documented. Numerous sources of electrical discharge have been cited as possible causes of catastrophic premature ignition of rocket motors containing solid propellants. External sources include natural lightning, electromagnetic pulses, high power microwave energy, exoatmospheric particle impact following launch, and the like.
In addition, static electricity charges are normally present at the interfaces between the various phases in the propellant, insulation, liner and other parts of the rocket motor. Charging of surfaces may occur by surface-to-surface, i.e., triboelectric contact and by the cracking or separation of the solid phase, as in fractoelectrification.
Sudden discharge of this electrostatic energy may generate sufficiently high temperatures to ignite the solid propellant. Such catastrophic events have the potential for causing great property damage and loss of life. For example, an incident in 1985 with a Pershing II missile killed two soldiers in Germany. Subsequent analysis indicated that triboelectric charging and an ensuing discharge was capable of initiating the unintentional ignition.
One manufacturing operation, which has been implicated as a cause of catastrophic discharge and premature propellant ignition, is the core pulling operation, i.e., removal of molds from the solid propellant grain after the grain is cast.
Other manufacturing operations have the potential for causing rapid electrostatic discharge. Such events may also occur during storage, transportation, and deployment of the rocket motor.
Composite solid propellants have a very complex microstructure consisting of a dense random pack of particles embedded in a polymeric binder matrix. The particles typically comprise fuel, oxidizers, combustion control agents, and the like. The particles may have a wide variety of sizes, shapes and electrical properties. Electrostatic charges typically build up on the binder-filler interfaces as well as at the interfaces between other components of the propellant, e.g., at the interface between conductive particles such as aluminum powder and a non-conductive or less-conductive binder.
The measurable electrostatic properties useful for evaluating binders and propellants include volume resistivity, surface resistivity, dielectric constant, dielectric breakdown strength, and relaxation time constant.
Volume resistivity and surface resistivity are measures of the propellant's ability to dissipate static energy imposed upon it. Mathematically, resistivity is the reciprocal of conductivity.
The dielectric constant is a measure of the propellant's ability to store electrostatic energy.
The dielectric breakdown threshold is a measure of how much electrostatic potential the propellant may be subjected to before a catastrophic breakdown occurs.
The relaxation time is the time taken by the propellant to dissipate an electrostatic charge.
Certain solid rocket propellants have a relatively high conductivity. For example, a propellant may contain HMX, i.e., Her Majesty's Explosive, ammonium perchlorate (AP) and aluminum in a binder of a nitrate ester plasticizer and polyethylene glycol (PEG). The latter complexes ammonium or alkali metal cations to dissolve several percent of the AP, theoretically providing a substantial population of dissociated ionic species available for charge transport. Thus, electrostatic charges are readily dissipated and catastrophic discharge is extremely unlikely with this type of propellant binder system.
In another propellant, the solid constituents are bound in a poybutadiene acrylonitrile/acrylic acid copolymer binder (PBAN). In this system, a quaternary benzyl alkyl ammonium chloride may be added to the binder polymer in its manufacturing process to ensure that the polymer will cure properly in the final propellant. The binder polymer contains polarizable functional groups along its nitrile “backbone.” The chemical structure of the polymer and the added quaternary ammonium salt are theorized to together result in a binder system with relatively high electrical conductivity. The relative contribution of each is unsubstantiated, however.
Quaternary ammonium salts have been used as antistatic agents in other industries to impart antistatic properties to fabrics, and in the manufacture of antistatic plastic products for sensitive electronic component manufacture.
In contrast to the particular fore-mentioned propellants using a PEG or PBAN binder, other propellants such as those using a hydroxyl-terminated polybutadiene (HTPB) binder have an intrinsic high insulative value, and are susceptible under certain circumstances to high charge build-up with accompanying catastrophic consequences resulting from breakdown discharge. Despite the dangers attendant to making and using such propellants, methods for reducing such dangers by modification of the propellant formulation have not been previously developed.